Investigate ASLR re-randomization being disabled for children

[Marc Deslauriers]

The systemd-socket-activation.patch patch has an Ubuntu delta to fix bug 2011458, but this results in ASLR not being re-randomized for children because the patch delta does "rexec_flag = 0;".

This was discovered as part of the CVE-2024-6387 discovery by Qualys, and is mentioned in the disclosure itself:

Side note: we discovered that Ubuntu 24.04 does not re-randomize the
ASLR of its sshd children (it is randomized only once, at boot time); we
tracked this down to the patch below, which turns off sshd's rexec_flag.
This is generally a bad idea, but in the particular case of this signal
handler race condition, it prevents sshd from being exploitable: the
syslog() inside the SIGALRM handler does not call any of the malloc
functions, because it is never the very first call to syslog().

This is also mentioned in the release notes of OpenSSH 9.8:

Exploitation on non-glibc systems is conceivable but has not been
examined. Systems that lack ASLR or users of downstream Linux
distributions that have modified OpenSSH to disable per-connection
ASLR re-randomisation (yes - this is a thing, no - we don't
understand why) may potentially have an easier path to exploitation.

We should investigate why that was needed, and if an alternative way of fixing the original bug can be done.

[Marc Deslauriers (mdeslaur)]

Subscribing Nick, who appears to be the original delta author.

[Julian Andres Klode (juliank)]

Assigning this to Nick so we don't lose track of it and can discuss it next week

[Seth Arnold (seth-arnold)]

These might be related and might help the underlying problem:

https://github.com/openssh/openssh-portable/commit/8785491123d4d722b310c20f383570be758f8263
https://github.com/openssh/openssh-portable/commit/c0cb3b8c837761816a60a3cdb54062668df09652

Thanks

[Nick Rosbrook (enr0n)]

I don't think those *help*, but that's good to know for when we merge an upstream version that includes those patches.